The present invention relates to an ion guide or mass filter device, a mass spectrometer, a method of guiding or mass filtering ions and a method of mass spectrometry. The preferred embodiment relates to a quadrupole rod set ion guide wherein a notched broadband frequency signal is applied to the rods of the quadrupole rod set ion guide. The notched broadband frequency signal preferably resonantly excites and radially ejects undesired background ions present in the ion guide whilst substantially unaffecting analyte or other ions of interest which are desired to be onwardly transmitted by the preferred ion guide or mass filter device.
A quadrupole rod set comprising four parallel rods is commonly used as an ion guide and as a mass filter or mass analyser. It is also known to use a quadrupole rod set as part of a linear ion trap wherein additional axial trapping potentials are applied in order to confine ions axially within the quadrupole rod set.
A quadrupole rod set comprising four parallel rods may be used as an ion guide to transmit ions without substantially mass filtering the ions by applying a two-phase RF signal or voltage to the rods. Adjacent rods are arranged to have opposite phases of the RF signal or voltage applied to them. The application of the RF signal or voltage to the rods results in a radial pseudo-potential well being generated which acts to confine ions radially within the quadrupole rod set. The four rods are maintained at the same DC potential or voltage. The quadrupole rod set ion guide may, in practice, exhibit a slight inherent low mass to charge ratio cut-off and the transmission efficiency of the ion guide may gradually reduce at higher mass to charge ratios. Nonetheless, to a first approximation at least the known quadrupole rod set ion guide may be considered as being arranged to transmit ions having a wide range of mass to charge ratios substantially simultaneously.
A quadrupole rod set may also be operated as a mass filter or mass analyser. According to this arrangement an RF signal or voltage is applied to the rods in a similar manner to a quadrupole rod set ion guide i.e. adjacent rods are supplied with opposite phases of a two-phase RF signal or voltage. However, instead of maintaining all the rods at the same DC voltage or potential, a DC component of voltage is applied to the rods such that adjacent rods have equal and opposite DC voltages applied to them. By applying an RF voltage to the rods and maintaining a DC potential difference between adjacent rods the quadrupole rod set can be arranged to act as a mass filter such that only ions having mass to charge ratios falling within well defined upper and lower mass to charge ratio cut-offs are transmitted by the mass filter. If the DC component is set to zero then the quadrupole rod set will then act as an ion guide in a non-resolving mode of operation wherein all ions received are substantially onwardly transmitted.
The mass to charge ratio transmission window of the mass filter can be narrowed to a point such that substantially only a single species of ion having a specific mass to charge ratio will be onwardly transmitted by the quadrupole rod set mass filter. All other ions will be substantially attenuated by the mass filter. Complete mass spectra can be obtained by scanning the RF and DC signals as a function of time so as to selectively sequentially transmit ions having different mass to charge ratios. The mass to charge ratio transmission window of the mass filter can therefore be progressively varied or increased. In this mode of operation the quadrupole rod set acts as a mass analyser.
A quadrupole rod set may also form part of a linear quadrupole ion trap. According to this arrangement an RF signal or voltage is applied to the rods in order to radially confine ions in a similar manner to a quadrupole rod set ion guide as described above. The rods are also all maintained at the same DC potential or voltage. In addition, potential barriers are maintained at the entrance and exit of the quadrupole rod set in order to prevent ions once they have been injected into the ion trap from exiting the quadrupole rod set ion trap in an axial direction. Ions are therefore effectively trapped within the quadrupole rod set ion trap. Once ions have been trapped within the ion trap, supplemental RF waveforms can then be applied to the electrodes of the ion trap in order to mass selectively eject certain ions either axially or radially from the ion trap. The frequency of the supplemental RF waveform which is applied to the electrodes can be scanned so as to mass selectively eject ions in sequence from the ion trap thereby enabling a mass spectrum to be produced. The resonance or first harmonic frequency ωr for ion excitation in a confining RF field is given by:
      ω    r    =            β      ⁢                          ⁢      Ω        2  wherein Ω is the angular frequency of the main confining RF voltage and β is a parameter related to the mass to charge ratio of an ion through the Matthieu stability parameters a and q.
Conventional quadrupole rod set mass filters will now be considered in more detail. As discussed above, both RF and DC voltage components are applied to a conventional quadrupole rod set mass filter. The quadrupole rod set mass filter may be operated so as to have a relatively wide mass to charge ratio transmission window and hence can operate as a relatively low resolution mass filter. Alternatively, the quadrupole rod set mass filter may be operated so as to have a relatively narrow mass to charge ratio transmission window in which case the quadrupole rod set mass filter may be considered as operating as a relatively high resolution mass filter.
Operating the mass filter in a relatively low resolution mode will obviously provide better specificity than simply operating the quadrupole rod set as an ion guide in a non resolving mode. However, when the mass filter is operated in a relatively low resolution mode then it will transmit in parallel a plurality of ions having a continuum of mass to charge ratios between upper and lower mass to charge ratio cut-off values of the mass to charge ratio transmission window.
Operating the mass filter in a relatively high resolution mode will provide better specificity but disadvantageously only a single species of analyte ion of interest can then be transmitted by the mass filter at any one time. Accordingly, if other species of analyte ions of interest are also present, then the overall duty cycle will be reduced. In order to analyse other species of analyte ions it is necessary to scan the mass to charge ratio transmission window of the mass filter in order to selectively transmit different analyte ions of interest in a sequential manner.
It is desired to provide an improved ion guide or mass filter device.